An electrically conducting fluid is a fluid containing mobile charge carriers. For example, liquid sodium and liquid gallium are electrically conducting fluids, the conductivity of which is of the order of 106 S·m−1, Plasmas are also conducting fluids. The measurement of the speed of flow of electrically conducting fluids is notably encountered in the iron and steel industry in the control of production lines. These measurements are also encountered in the liquid metal cooling circuits of nuclear power stations. They can equally be encountered in plasma processes (depollution, etching or deposition of thin layers),
The applicant knows of a velocimeter including;                a primary magnetic field source, the field lines of which pass through the electrically conducting fluid, and        first and second magnetic field sensors each measuring the intensity of the magnetic field in the same first measurement direction.        
By measurement direction is meant a set of collinear vectors oriented in the same direction.
In the absence of flow of the fluid, the field lines of the primary magnetic field source have an initial shape. The sensors then measure an intensity of the magnetic field at zero speed of flow. When the fluid is flowing, the movement of the mobile charge carriers deforms the field lines passing through the fluid. The intensity measured by the sensors therefore varies with the speed of flow of the fluid. To increase the sensitivity of the measurements, in known velocimeters, the first and second sensors are disposed symmetrically with respect to a plane of symmetry of the primary magnetic field source and the intensities measured by these two sensors are added. A plane of symmetry of the primary magnetic field source is a plane of symmetry for the lines of the primary magnetic field.
Moreover, the sensors are placed as close as possible to the source so as to be more sensitive to the primary magnetic field than to other magnetic fields. Despite these precautions, known velocimeters remain sensitive to interference caused by an exterior magnetic field such as the terrestrial magnetic field or a magnetic field radiated by nearby electrical equipment. Moreover, the linearity of known velocimeters is open to improvement.
By “linearity” is meant the maximum difference between the ideal linear characteristic curve that links the measured speed to the output signal of the velocimeter and its real characteristic curve.
The invention aims to resolve one or more of these drawbacks.
It therefore consists in a velocimeter for measuring the speed of flow of an electrically conducting fluid in which:                the two sensors are disposed so that, in the absence of flow of the electrically conducting fluid, the difference between the intensities measured by the two sensors is not zero, and        the velocimeter includes a processing unit adapted to calculate the speed of flow of the fluid in a direction parallel to the first measurement direction from the difference between the intensities measured by the two sensors.        
Using the difference between the measured intensities makes it possible to improve the linearity of the velocimeter, as explained hereinafter. Moreover, this differential measurement makes it possible to limit the dependence of the measured speed on:                temporal variations of the exterior magnetic field,        the field induced by speed gradients in the presence of the exterior magnetic field, and        space and time variations of the magnetic field induced by the gradients.        
Embodiments of this velocimeter may have one or more of the following features:                the two sensors are disposed so that, in the absence of flow of the electrically conducting fluid, the difference between the intensities of the primary magnetic field measured by the two sensors is greater than the smaller of those intensities,        the source has a magnetization axis and the two sensors are disposed on the same side of a plane containing the magnetization axis,        the first sensor is situated at a distance D16 from the primary magnetic field source and the second sensor is situated at a distance D191 from the primary magnetic field source so that the absolute value of the difference between the distances D16 and D191 is greater than the smaller of the distances D16 and D191,        a magnetic disturber is disposed closer to one sensor than the other sensor so that in the absence of flow of the fluid, the intensity measured by the first sensor is at least twice the intensity measured by the second sensor,        the first and second sensors are disposed at equal distances from the primary field source,        the velocimeter includes a third magnetic field sensor, the third sensor being adapted to measure the magnetic field in the first measurement direction, the third sensor being disposed relative to the source so that the intensities measured by the first sensor and the third sensor are equal in the absence of flow, and the processing unit is adapted to calculate the speed of flow of the fluid from:                    the difference between the intensities measured by the first and second sensors,            the difference between the intensities measured by the second and third sensors, and            the sum of these differences,                        the primary magnetic field source is an alternating magnetic field source,        the primary magnetic field is modulated by a predetermined modulating signal and the intensities measured by the sensors are demodulated with the aid of that modulating signal,        the velocimeter includes a fourth magnetic field sensor and a fifth magnetic field sensor measuring the intensity of the magnetic field in at least one second measurement direction intersecting the first measurement direction, and the processing unit is adapted to calculate the speed of flow of the fluid in the second measurement direction of the fourth and fifth sensors from the difference between the intensities measured by the fourth and fifth sensors.        
These embodiments of the velocimeter also have the following advantages:                placing the first and second sensors so that the difference between the intensities of the primary magnetic field measured by these sensors is greater than the lower of these intensities makes it possible to improve the linearity of the velocimeter whilst preserving the sensitivity of the sensor nearest the source;        placing the two sensors on the same side makes it possible to move them closer together and to compensate more effectively the non-linearity caused by the gradient of the speed of the flow;        adding a magnetic disturber makes it possible to produce more compact velocimeters since it is then no longer necessary to place the second sensor far from the source so that it measures a primary magnetic field intensity that is negligible compared to the intensity of the primary magnetic field measured by the first sensor;        adding a third sensor measuring the same magnetic field as the first sensor makes it possible to increase the sensitivity of the velocimeter;        modulation of the primary magnetic field radiated by the source, coupled with demodulation of the intensity measured by the sensors, makes it possible to set up synchronous detection limiting the consequences of noise;        adding third, fourth and fifth two-axis sensors makes it possible to measure the speed of flow of the fluid in three non-colinear directions in space.        